If you are painting the back of your aquarium, ON THE OUTSIDE (it should always be done on the outside) use fusion. It's safe enough to be used on the outside. I wouldn't call it "reef safe". I would think it would crash a tank if sprayed into it. It's a ton of chemicals.
If not the back, then are you painting a false back or grates or something?
I am painting the inside of a larval grow out tub for baby clowns. I have tried pond liner but am having problems with the rubber releasing toxins. From what I have read, krylon fusion is supposed to be aquarium safe.
I have used krylon fusion and another off-brand spray acrylic for items to be placed IN the tank before. My DT has pvc-frame rock columns that were painted with fusion, and it's going on 2 years now without any measurabe deletrious effects. Just be sure you let whatever item you've painted DRY for at least 24 hours before you go putting it in the tank, and honestly the longer you let it dry, the better. I waited 7 days for the frames to dry in my DT, and another set of rock columns that went into another tank got a 3-day drying time. Basically you ant to make sure that any VOC's that might be present in the paint to evaporate off. IMHO Krylon Fusion is very reef-safe.
Krylon Fusion/Rusto Plastics are 100% reef safe when cured over 24 hours. No need to cur in water or anything, that is just a waste of time. I have several things in my SPS dominated reef painted black and have never had a problem.
You can also use this paint to seal plastics that would otherwise leak toxins into your tank. i.e... eggcrate, DIY HOB stuff etc etc.
Hope that helps.